Magnetic shear effects on ballooning turbulence in the boundary of fusion devices

Conditions for the excitation of Alfvén eigenmodes (AEs) by energetic ions are investigated in neutral-beam-injection (NBI) heated plasmas of the Large Helical Device (LHD). This study is carried out in a wide parameter range of the beta values of the energetic ion components and the ratio of the energetic ion velocity to the Alfvén velocity (up to with the assumption of classical slowing down and ). These ranges of parameters cover those predicted for the International Thermonuclear Experimental Reactor (ITER). During this experimental campaign of LHD, toroidicity-induced AEs (TAEs) with n = 1–5 (n being the toroidal mode number), global AEs (GAEs) with n = 0 and 1, and energetic particle modes (EPMs) were observed. The effect of the magnetic configuration on the TAE spectrum was also investigated. In magnetic configurations with relatively high magnetic shear, only TAEs with n = 1 and 2 were observed. On the other hand, TAEs with n up to 5 were observed in magnetic configurations with low magnetic shear. For two typical shots obtained in magnetic configurations characterized by different values of the magnetic shear, eigenfunctions of TAEs were calculated by using a global mode analysis code CAS3D3. The calculated results indicate that the eigenfunctions tend to be localized around the relevant TAE gaps. When the gap is located in the plasma core region (normalized minor radius ρ ⩽ 0.4), the TAE tends to become a core-localized type. When the gap is in the outer region (typically 0.5 ⩽ ρ ⩽ 0.9) of the plasma, the TAE tends to (a) either become a global type having a radially extended structure if the magnetic shear is very weak in the core region inside the gap, (b) or become a gap localized type in the case of finite central magnetic shear. Transition of the eigenmode from the core-localized type with m ∼2/n = 1 TAEs (m being the poloidal mode number) to the n = 1 GAEs (or cylindrical AEs) has been observed when the rotational transform at the core ι (0)/2π exceeds the specific value of ι(0)/2π = 0.4.

Magnetic shear and flow shears form across Earth’s magnetopause when shocked solar winds flow around Earth. Previous studies have shown that these two kinds of shears can similarly affect magnetopause reconnection. However, a direct investigation to evaluate their relative importance is lacking. In this study, we focus on simultaneous magnetopause reconnection observed by Magnetospheric Multiscale mission (MMS) and Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft at different magnetopause locations to quantitatively evaluate the magnetic shear and flow shear effects. In these observations, the overall effect of magnetic shear (the normalized guide field < 1) is limited unless the guide field is large enough to suppress reconnection, while the flow shear can significantly affect the observed reconnection outflow speed primarily by introducing non-zero X-line motion. Finally, we propose a new relationship combining magnetic and flow shear effects by assuming the X-line drift motion is independently affected by these two effects, which shows that X-line drift speed is dominated by the magnetosheath flow, and the suppression of reconnection is more likely to occur under strong guide field conditions. This study deepens our understanding on magnetopause reconnection occurrence and reconnection behaviors in large scales.

A spectral filament model for turbulent transport and scrape off layer width in circular geometry

Ion temperature gradient (ITG or ηi) driven microinstabilities are studied, using fluid and kinetic theories, for plasmas with ion temperature and temperature gradient anisotropy. The sheared slab geometry model (nonlocal scheme) is employed. The effects of a parallel velocity shear and a perpendicular velocity shear on the modes are investigated. It is shown that the anisotropy in ion temperature gradient enhances (reduces) the stabilization from a magnetic shear for η⊥>η∥ (η⊥<η∥). An anisotropy of T⊥>T∥ in ion temperature is found to give an overall stabilizition (destabilization) for low (high) magnetic shear, ŝ∼0.1 (ŝ∼0.4). Parametric dependence of the instabilities is systematically analyzed. Previous results from the shearless toroidal model are confirmed with a sheared torus.

Magnetic shear and flow shear form across Earth's magnetopause when shocked solar winds flow around Earth. Previous studies have shown that these two kinds of shears can similarly affect magnetopause reconnection. However, a direct investigation to evaluate their relative importance is lacking. In this study, we focus on simultaneous magnetopause reconnection observed by Magnetospheric Multiscale mission and Time History of Events and Macroscale Interactions during Substorms spacecraft at different magnetopause locations to quantitatively evaluate the magnetic shear and flow shear effects. The overall effect of magnetic shear (the normalized guide field < 1) is limited unless the guide field is sufficient strong to suppress reconnection, whereas the flow shear can significantly affect the observed reconnection outflow speed primarily by introducing non‐zero X‐line motion. Finally, we propose a novel relationship combining magnetic and flow shear effects by assuming independent X‐line drift motion from these two effects, which shows that the X‐line drift speed is dominated by the magnetosheath flow, and the suppression of reconnection is more likely to occur under strong guide field conditions. This study deepens our understanding on magnetopause reconnection occurrence and reconnection behaviors in large scales.

Dedicated transport experiments based on q-profile modulation have been performed in EAST with broadband turbulence dominated plasmas. Reduced broadband turbulence under lower internal inductance (li) is observed by a CO2 laser collective scattering system in a series of experiments across different current platforms. Experimental observations found that under higher current platforms, the internal inductance decreases with weaker magnetic shear, leading to a significant decrease in turbulence (for 1 ≤ kθρs ≤ 5) and improved confinement, which is consistent with power balance analysis. The stabilizing effect of low magnetic shear under low li on turbulence and thermal transport is also verified by using the gyrokinetic code NLT. Linear simulations show that both the growth rate and frequency of trapped electron mode under low magnetic shear are smaller than those under high magnetic shear. Nonlinear simulations are also conducted and compared to experimental results, which indicate that the reduction in electron turbulent transport with the low shear q profile may have contributed to improve Te. This work contributes to understanding the impact of magnetic configurations on turbulence transport.

We address the conditions for the onset of magnetic reconnection based on a survey of 197 reconnection events in solar wind current sheets observed by the Wind spacecraft. We report the first observational evidence for the dependence of the occurrence of reconnection on a combination of the magnetic field shear angle, θ, across the current sheet and the difference in the plasma β values on the two sides of the current sheet, Δβ. For low Δβ, reconnection occurred for both low and high magnetic shears, whereas only large magnetic shear events were observed for large Δβ: Events with shears as low as 11° were observed for Δβ 1.5 only events with θ > 100° were detected. Our observations are in quantitative agreement with a theoretical prediction that reconnection is suppressed in high β plasmas at low magnetic shears due to super-Alfvenic drift of the X-line caused by plasma pressure gradients across the current sheet. The magnetic shear-Δβ dependence could account for the high occurrence rate of reconnection observed in current sheets embedded within interplanetary coronal mass ejections, compared to those in the ambient solar wind. It would also suggest that reconnection could occur at a substantially higher rate in solar wind current sheets closer to the Sun than at 1 AU and thus may play an important role in the generation and heating of the solar wind.

Intensity fluctuations are investigated using the fast camera imaging technique in the slab annular plasma as a function of magnetic shear and connection length in the spherical tokamak QUEST. Note that here QUEST is operated as a simple magnetized torus with a tight aspect ratio. Slab annular plasmas feature open magnetic field lines and can mimic the tokamak edge-scrape off layer (SOL)-like plasma attributes reasonably well. Three magnetic shear regimes are realized using three poloidal magnetic field (PF) coil pairs. A whole range of connection lengths (∼∞ ≥ Lc ≥ 5.5 m) is scanned by varying the PF strength for a given toroidal field for each magnetic shear regime. For the first time a systematic study of the effect of magnetic shear and field line pitch together on edge-SOL-like plasma fluctuations is being reported. Slab plasmas with intermediate magnetic shear are observed to be more susceptible to generate distinct blobs when Lc is reduced by increasing the PF strength. A distinct coherent mode appears only at the lowest magnetic shear slab featuring a deep potential well. Such mode is not apparent at other magnetic shear cases even at the same Lc. Finally, with a combination of PF coil pairs, both the features of intermediate and low magnetic shear slabs are shown to be realizable simultaneously. Significantly stronger blobs are observed with such combination of PF mirror ratios in the presence of a coherent mode. This study may provide better insight into the effect of magnetic configuration in the tokamak edge and SOL turbulence and can help in searching for better tools to control cross-field convective intermittent transport in tokamaks.

Effects of magnetic shear on electron cyclotron resonance heating (ECRH) are studied in heliotron/torsatron configurations. In such configurations, the poloidal magnetic field is comparable to the toroidal magnetic field, and varies spatially along the minor radius, making a strong magnetic shear. When high power millimeter waves are launched into a plasma, it is coupled to propagating modes at the plasma peripheral region. The existence of a transition layer between the core plasma region and the vacuum region, where the magnetic field direction is largely changed, requires accurate polarization control for good single pass absorption. The mode conversion between the propagation modes due to the magnetic shear also affects the launching conditions. The polarization control experiment by using second harmonic ECRH in Heliotron E [T. Obiki, Fusion Technol. 17, 101 (1990)] are compared with the numerical calculation in which one dimensional second order coupled equations are solved. The polarization dependence experimentally measured is in good agreement with the numerical results including the magnetic shear terms.

Summary form only given. A global normal mode stability analysis on the effect of both magnetic shear and flow shear on magnetohydrodynamic (MHD) instabilities in Z-pinch plasmas is presented. A general set of equations based on the Hall fluid model allows the investigation of ideal and non-ideal MHD plasmas with both magnetic shear and flow shear included in the plasma equilibrium. These equations are integrated numerically by following the linear development in time of an initial see perturbation. Global instability growth rates are obtained after the numerical solution converges to the fastest growing mode. Various equilibrium profiles with radially sheared azimuthal flow and/or radially sheared axial magnetic field are analyzed. Comparisons of instability growth rates for a broad range of shear amplitudes are reported.

Intrinsic torque, which can be generated by turbulent stresses, can induce toroidal rotation in a tokamak plasma at rest without direct momentum injection. Reversals in intrinsic torque have been inferred from the observation of toroidal velocity changes in recent lower hybrid current drive (LHCD) experiments. This work focuses on understanding the cause of LHCD-induced intrinsic torque reversal using gyrokinetic simulations and theoretical analyses. A new mechanism for the intrinsic torque reversal linked to magnetic shear ( ŝ) effects on the turbulence spectrum is identified. This reversal is a consequence of the ballooning structure at weak ŝ. Based on realistic profiles from the Alcator C-Mod LHCD experiments, simulations demonstrate that the intrinsic torque reverses for weak ŝ discharges and that the value of ŝcrit is consistent with the experimental results ŝcritexp≈0.2∼0.3 [Rice et al., Phys. Rev. Lett. 111, 125003 (2013)]. The consideration of this intrinsic torque feature in our work is important for the understanding of rotation profile generation at weak ŝ and its consequent impact on macro-instability stabilization and micro-turbulence reduction, which is crucial for ITER. It is also relevant to internal transport barrier formation at negative or weakly positive ŝ.

The effect of magnetic shear on plasma transport for an electron to ion temperature ratio (T e/T i) near unity has been explored in DIII-D utilizing electron cyclotron heating (ECH). Previous reports showed that significant confinement degradation occurred at T e/T i ∼ 1 in positive shear (PS) plasmas in DIII-D, whereas reduced confinement degradation was observed in negative central shear (NCS) plasmas. In this study, plasma transport in weak magnetic shear (WS) plasmas with ECH is investigated and compared with that in NCS and PS plasmas. Here the magnetic shears () are > 0.5, ∼0 and <-0.1 in the core region (ρ∼ 0.3–0.4) of PS, WS and NCS plasmas, respectively, and flat or negative inside ρ∼ 0.4 in the WS and NCS plasmas. Weak magnetic shear is found to be effective in minimizing degradation of ion thermal confinement as T e/T i increases through ECH application, and an improved confinement factor of H 98y2 ∼ 1.2 is maintained, similar to NCS plasmas. At T e/T i ∼ 1, the ion thermal diffusivity around an internal transport barrier decreases when changing the magnetic shear from positive to weak or negative shear. Also, reduced local particle and momentum transport was indicated by steeper density and toroidal rotation profiles in the weak and negative shear regimes. Linear gyrokinetic simulations predict little change in growth rates of low-k turbulence with ECH application in the WS and NCS plasmas, which is consistent with the transport and profile analyses.

Cascades of high-n tearing modes with mode bursts in order of increasing n number have recently been observed in ASDEX Upgrade in the vicinity of the q = 1 surface. The relationship between the poloidal and toroidal mode numbers is m = n + 1. A cascade commences with a low n number, typically , and can reach values around 22. They are observed in discharges with impurity accumulation and with a transient evolution of the q profile, where the magnetic shear is low and the resistivity high. Calculations with the CASTOR code have shown that high-n tearing modes become unstable if the magnetic shear is low, resistivity is high and the pressure gradient high enough to make mode coupling important. All three conditions are needed to obtain a positive and large growth rate. These CASTOR code calculations are consistent with the experimental observations.

Introduction: &amp;#160;Kelvin-Helmholtz instability (KHI) is considered one of the main processes of transferring solar wind energy, momentum and plasma inside magnetosphere. At the Earth, KHIs are observable in the form of waves (KHWs) within the magnetopause region between the anti-Sunward magnetosheath and the relatively stagnant magnetosphere.&amp;#160;Over the past few decades, several missions (THEMIS, Cluster, MMS) have contributed significantly to our understanding of KHI. In this sense, it is well-known that (i) KHIs are a common phenomenon [1,2], (ii) KHIs can be generated under different IMF conditions [2], (iii) KHIs can lead to rolled-up vortices [3], (iv) KHVs drive the onset of magnetic reconnection (Vortex-Induced Reconnection) leading to development of Tearing Mode instability [4] and formation of magnetic islands, evolving into flux ropes [5], and (v) in the late nonlinear phase, vortex merging and secondary KHIs development in a wider latitudinal range [6]. In addition, [7] found that KHWs occur at the (flank) magnetopause for approximately 19% of that time. The occurrence of these waves is influenced by factors such as solar wind speed, Alfven Mach number and number density, and is mostly independent on the IMF magnitude. These conditions can be easily met when a perturbation propagates within the interplanetary medium, such as during the occurrence of a coronal mass ejection.&amp;#160;&amp;#160;Investigating the conditions under which KH and TM instabilities occur in the Earth environment, using simultaneous multipoint in-situ measurements and MHD simulations, is intriguing because it could provide insights into the flow dynamic nature at the magnetopause mixing layer. In this sense, we analyzed data from THEMIS and Cluster spacecraft considering two "target" Space Weather events occurred on 21 June 2015 (Case-1) and on 6 September 2017 (Case-2).&amp;#160;The Model: Our analysis utilized a 2D MHD model [8] which describes the flow dynamics of the magnetopause mixing layer in a fluid limit.&amp;#160; The simulation domain consists of a rectangular region in (x,y)-plane. On the local Cartesian grid, it is defined as follows: (i) neglecting the realistic curvature; (ii) considering the x-coordinate pointing to the direction along the velocity of the incident magnetosheath flow; (iii) assuming the y-coordinate in the direction downward to the Earth&amp;#8217;s center (from the magnetosheath to the magnetosphere) and (iv) ensuring a right-handed coordinate system with the z-coordinate.The used approach is flexible enough to represent any position on the dayside magnetopause.&amp;#160;Results: In Case-1, we used our MHD model to interpret observational data and to investigate the potential development of KHVs on the dawn flank magnetopause as a consequence of the arrival of the ICME. Using THEMIS-E data, we found that at the magnetopause nose no rolled-up KHVs developed due to the absence of a shear between the two fluids. On the contrary, structures similar to magnetic islands appeared in By component very fast and vanished for 10 computational seconds. At the dawn flank magnetopause, the analysis of Cluster data revealed high flow and low magnetic shear between the magnetosheath and the magnetosphere. According to theoretical predictions, these conditions favour the onset of KHI. MHD simulations confirmed these considerations, finding that KHVs developed very rapidly and persisted up to 20 computational seconds (Figure 1), reaching almost MHD instability steady state. Regarding the TM instability, the MHD simulations revealed only an early development of magnetic islands (Figure 2), that persisted for half of the time of the KHVs evolution. In a global scale, these results indicate that vortices become unstable far away from the subsolar point in the direction of high flow shear. Figure 1. Case 1. Evolution of KH instabilities in density at the dawnward flank of the magnetopause, from 1 to 30 seconds. Input data obtained from the Cluster-C4 measurements. The dimensionless boundary conditions from the magnetosphere side and magnetosheath side are taken everywhere to be identical: BMSPx=-1.24, BMSHx=1.0, BMSPy=0.4, BMSHy=1, &amp;#961;MSP =0.2, &amp;#961;MSH=1, Re= 250 and Rm= 1 000, MA= 0.2. Blue region represents the magnetosphere whilst red region represents the magnetosheath.&amp;#160;Figure 2. Case 1.&amp;#160; Evolution of TM instabilities in By at the dawnward flank of the magnetopause. Same of Figure 1.&amp;#160;In Case-2, using THEMIS-E data, we did not find any evidence of KHI owing to the extremely low flow and high magnetic shear. On the contrary, adopting Cluster-C4 data, MHD simulations revealed that the fast development of disturbances but no signatures of KHVs were visible. Additionally, magnetic islands appeared very fast as a result of high shear in the components of the magnetic field but rapidly vanished.&amp;#160;&amp;#160;

The evolution of a local helical perturbation and its stability property for arbitrary magnetic shear configurations are investigated for the case of in cylindrical geometry. An analytic stability criterion has been obtained which predicts that a strong magnetic shear will enhance the instability in the positive shear region but enhance the stability in the negative shear region. The perturbations with the poloidal and toroidal perturbation mode numbers m/n = 1/1 is most unstable due to the stabilizing terms increasing with m. For m/n = 1/1 local perturbations in the conventional positive magnetic shear (PMS) configurations, a larger qmin exhibits a weaker shear in the core and is favourable to the stability, while in the reversed magnetic shear (RMS) configurations, a larger q0 corresponds to a stronger positive shear in the middle region, which enhances the instability. No instabilities are found for m≥2 local perturbations. The stability for RMS configuration is not better than that for PMS configuration.